1. Field
Exemplary embodiments of the present invention relate generally to an auto-focus technology for a digital imaging device and more particularly, an auto-focus system and method based on a sensor using hierarchical feature extraction and motion estimation for an image.
2. Description of the Related Art
The auto-focus technology is one of the most important technologies for acquiring a clear image in an image acquisition process of a digital imaging device, such as a camera.
The auto-focus technology may be generally classified in active methods which directly measure a distance using an additional signal, and passive methods which measure a distance by analyzing light transmitted through a lens.
Active methods generally employ a module for auto-focus, and calculate a focal distance based on the time required for a specific signal which is emitted by the camera to return to the camera after it is reflected on an object. The passive method calculates a distance by analyzing a light source of a visible area, which is reflected from an object and transmitted to the camera.
The passive method may be generally divided into a Contrast Detection Auto-Focus (CDAF) method and a Phase Detection Auto-Focus (PDAF) method. All of the auto-focus methods are currently applied to digital cameras.
Recently, much attention has been paid to a hybrid auto-focus method which combines the advantages of the active method and the passive method.
The hybrid auto-focus method primarily determines an in-focus state using the active method or the PDAF method, and secondarily provides a clear in-focus image using the CDAF method which generally may provide better precision.
In the hybrid auto-focus method, the primary auto-focus method is very important in determining the computing speed of the auto-focus system. That is because, in the primary auto-focus method, the lens needs to be as close as possible to the in-focus state, in order to minimize repetitive motions of the lens in the CDAF method which is used as the secondary auto-focus method.
For the hybrid auto-focus method, research has been recently conducted on a focus method in which pixels for calculating a phase difference are mounted in an imaging sensor.
In this method, two measuring point pixels used for auto-focus are covered with a black mask so as to receive different phases.
However, due to the black mask which is separately mounted on the pixels in order to define different phases, the amount of light received by the measuring point pixels is smaller than the amount of light received by other pixels, and out-of-focus blur exists in an image inputted to the camera.
Under such a condition, it is very difficult to detect a feature and to acquire a reliable phase difference. Thus, there is a demand for a method which is capable of extracting a reliable phase difference image, calculating an accurate phase difference through the phase difference image, and estimating a motion.
Japanese Patent Laid-Open Publication NO. 2014-182237, Japanese Patent Laid-Open Publication NO. 2013-152388, US Publication NO. 2013-0271646, US Publication No. 2013-0235253 and US Publication No. 2012-0133821 describe generally autofocus systems for digital cameras.